Related Stories

Researchers in the UK, Australia and the Czech Republic have observed that in wood mice, sperm join together to speed their route to the egg.

Sperm 'trains' of hundreds of thousands of individuals start forming just after ejaculation, report the scientists in Nature. The sperm link together using a hook on their heads, or by grabbing the tail of the sperm in front.

Since groups of sperm travel twice as fast as individual sperm, working together like this gives them an advantage over sperm from other males.

Professor Harry Moore of the University of Sheffield and his team describe how these sperm trains swim en masse towards the egg, before starting to fall apart about half an hour later.

The sperm commit genetic hara-kiri by undergoing a premature 'acrosome reaction'. This involves the release of enzymes that break down cell adhesion molecules, with the result that the sperm cannot fertilise the egg.

Only one  usually the 'train driver' up front  remains unaffected by the mass suicide and retains this ability.

The mating system of any animal is related to size of its testes compared to its body weight. The wood mouse is a multi-male mater, and has developed relatively large testes for producing lots of sperm that can out-complete those of its rivals.

Species with relatively small testes, such as humans, are not multi-male maters.

Significantly, this study shows that the male wood mouse does not try to ensure his reproductive success through sheer sperm numbers alone, but by sperm collaboration as well.

This is the first time altruistic cooperation to this extent has been confirmed in any species.

Sperm in most South American marsupials swim in pairs that split up just before fertilisation. This is also presumed to give some advantage, though less than that demonstrated by the wood mouse's conga-line.

Professor Roger Short, from the University of Melbourne, said that the finding puts paid to past speculation  most notably by Robin Baker in his book Sperm Wars  that within the ejaculate, sperm fight each other to reach the egg.

"It's now much more likely that all the sperm in one ejaculate help each other so that at least one of them gets there," he said.

"Here is the supreme example of a whole lot of sperm altruistically giving up their own chance of ever fertilising an egg and shoving along somebody else of your own genotype who will make it."

Professor Short discovered the testes sizebody weight ratio in mammals, and has had a longstanding interest in the consequences of the multi-male mating system.